Wireless communication systems enable users of User Equipment (UE) to communicate with other such users via one of a number of base stations and via one or a number of core networks. Typically, the UEs are mobile terminals, such as mobile (cellular) telephones and the like, although the term UE may also refer to generally stationary communication devices, such as laptop computers, web browsers, machine-type communication devices, and the like. In the following description the term user communication device is used, which is intended to cover any type of such user equipment (mobile and stationary).
In an active or connected state a user communication device is registered with the network and has a Radio Resource Control (RRC) connection with a base station so that the network knows to which base station (or cell thereof) the user communication device belongs and can transmit data to and receive data from the user communication device. Each user communication device also establishes a default Evolved Packet System (EPS) Bearer (i.e. an end-to-end dedicated communication path) from the user communication device to an endpoint beyond the base station, typically a gateway (such as a packet data network gateway—‘PDN-GW’ or ‘P-GW’—or the like), in the Enhanced Packet Core (EPC) network, or core network for short. An EPS Bearer, which is specific to the user communication device, defines a transmission path for IP packets through the network. The EPC assigns one or more IP addresses to the mobile communication device, at which it can be reached by other communication devices, such as another mobile communication device.
A Mobility Management Entity (MME) in the core network manages general mobility aspects of the user communication devices and ensures that connectivity is maintained with the user communication devices, for example as they are moving within the geographical area covered by the communication system (and/or as they are handed over between base stations of the communication system due to movement or changes in communication conditions). The MME also manages the various bearers associated with the user communication devices (such as an EPS bearer and/or the like) by controlling the other network nodes (e.g. P-GW, S-GW) via which such bearers are provided. In order to do so, the MME exchanges Non-Access Stratum (NAS) signalling messages with the user communication devices (and/or the other network nodes) to manage the establishment of a communication session.
As part of the Long Term Evolution (LTE) of UTRAN (UMTS Terrestrial Radio Access Network) referred to as E-UTRAN, proximity-based services (ProSe) have been introduced, which make use of direct device-to-device (D2D) communication bearers directly between compatible user communication devices rather than indirect bearers provided from one user communication device to another user communication device, via a base station and the core network (or via a base station when using locally routed indirect bearers), e.g. over a pair of EPS bearers (or over a pair of locally routed radio bearers). Thus, when a ProSe enabled user communication device is within the transmission range of (or served by the same base station as) another ProSe user communication device, they can communicate user data without the need to use core network resources and/or base station resources. Such services can be achieved by establishing a special, ‘D2D’, bearer between the user communication devices in direct communication instead of their default or other conventional EPS bearers (which might be still used for other types of communications). This direct routed communication could result in better utilization of the available resources, especially on the radio interface, where these are limited. Details of the ProSe functionality have been specified in the 3GPP Technical Report TR 23.703 and 3GPP Technical Specification TS 23.303 documents, the contents of which are incorporated herein by reference.
More recently the provision of a relay functionality, in the user communication device, using the ProSe functionality has been proposed to allow one user communication device (referred to as a ‘UE-Relay’ or ‘UE-R’) to relay the signalling and the user data for another user communication device to and from the network, even if the other user communication device is not located within the network's coverage. In this case, the relayed user communication device can communicate with the network (both user plane and control plane data) via the UE-R thus accessing to network services as if the relayed user communication devices were served by a base station of the network.